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United States Patent |
5,534,750
|
Theiss
,   et al.
|
*
July 9, 1996
|
Integral polepiece magnetic focusing system having enhanced gain and
transmission
Abstract
A focusing system for an electron beam within an RF amplification tube is
provided. The focusing system comprises a plurality of magnetic polepieces
each having a centrally disposed aperture, and a plurality of electrically
conductive non-magnetic plates alternatingly and integrally provided with
the polepieces, the non-magnetic plates each having a centrally disposed
aperture. The apertures of the polepieces are aligned with the apertures
of the non-magnetic plates to provide a beam tunnel through which the
electron beam travels. At least one permanent magnet is coupled to the
polepieces, the magnet having magnetic flux which flows through the
magnetic polepieces to provide an axial magnetic field within the beam
tunnel. The diameter of the beam tunnel is selected to be greater than a
separation distance between adjacent ones of said polepieces, and the
axial magnetic field varies substantially across a cross section of the
beam tunnel. The axial magnetic field has a greatest RMS value at an
outermost portion of the beam tunnel.
Inventors:
|
Theiss; Alan J. (Redwood City, CA);
Lyon; Douglas B. (San Carlos, CA)
|
Assignee:
|
Litton Systems, Inc. (Woodland Hills, CA)
|
[*] Notice: |
The portion of the term of this patent subsequent to July 6, 2011
has been disclaimed. |
Appl. No.:
|
263762 |
Filed:
|
June 22, 1994 |
Current U.S. Class: |
315/5.35; 315/39.3; 335/210; 335/306 |
Intern'l Class: |
H01J 023/087 |
Field of Search: |
315/5.35,39.3
335/210,306
|
References Cited
U.S. Patent Documents
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|
3099765 | Jul., 1963 | Meyerer | 315/3.
|
3188533 | Jun., 1965 | Bretting et al. | 335/210.
|
3711943 | Jan., 1973 | James | 29/600.
|
3755706 | Aug., 1973 | Scott | 315/5.
|
3989978 | Nov., 1976 | Souseng et al. | 315/3.
|
3993924 | Nov., 1976 | Hanf | 315/3.
|
4103207 | Jul., 1978 | Chaffee | 315/3.
|
4137482 | Jan., 1979 | Caryotakis et al. | 315/5.
|
4399389 | Aug., 1983 | Fleury et al. | 315/5.
|
4409519 | Oct., 1983 | Karp | 315/3.
|
4560904 | Dec., 1985 | Wolfram | 315/3.
|
4578620 | Mar., 1986 | James et al. | 315/3.
|
4586009 | Apr., 1986 | James | 333/156.
|
4619041 | Oct., 1986 | Davis et al. | 29/600.
|
4800322 | Jan., 1989 | Symons | 315/3.
|
4931694 | Jun., 1990 | Symons et al. | 315/5.
|
4931695 | Jun., 1990 | Symons | 315/5.
|
4942336 | Jul., 1990 | Amboss et al. | 315/535.
|
5304942 | Apr., 1994 | Symons et al. | 330/45.
|
5332947 | Jul., 1994 | Theiss et al. | 315/3.
|
5332948 | Jul., 1994 | True et al. | 315/5.
|
Foreign Patent Documents |
1233065 | Jan., 1967 | DE.
| |
2168537 | Jun., 1990 | JP | 315/5.
|
742070 | Dec., 1955 | GB.
| |
1048440 | Nov., 1966 | GB.
| |
1053861 | Jan., 1967 | GB.
| |
1140917 | Jan., 1969 | GB.
| |
Primary Examiner: Lee; Benny T.
Attorney, Agent or Firm: Graham & James
Parent Case Text
RELATED APPLICATION
This is a continuation-in-part of application Ser. No. 07/882,298, filed
May 13, 1992, for INTEGRAL POLEPIECE RF AMPLIFICATION TUBE FOR MILLIMETER
WAVE FREQUENCIES, issued as U.S. Pat. No. 5,332,947 on Jul. 26, 1994.
Claims
What is claimed is:
1. An integral polepiece focusing structure for an RF amplification tube,
comprising:
a slow-wave circuit comprising a plurality of magnetic polepieces and a
plurality of electrically conductive non-magnetic plates which are
alternatingly and integrally coupled into a laminate structure;
a means for inducing a magnetic field in said slow-wave circuit having
lines of flux which flow through said magnetic polepieces; and
a beam tunnel provided through said structure, said magnetic polepieces
extending substantially entirely to said beam tunnel;
wherein a diameter of said beam tunnel is greater than a separation
distance between adjacent ones of said polepieces.
2. The focusing structure of claim 1, wherein said magnetic field induced
by said inducing means has an axial RMS value that varies substantially
across a cross section of said beam tunnel.
3. An integral polepiece focusing structure for an RF amplification tube,
comprising:
a slow-wave circuit comprising a plurality of magnetic polepieces and a
plurality of electrically conductive non-magnetic plates which are
alternatingly and integrally coupled into a laminate structure;
a means for inducing a magnetic field in said slow-wave circuit having
lines of flux which flow through said magnetic polepieces; and
a beam tunnel provided through said structure, said magnetic polepieces
extending substantially entirely to said beam tunnel;
wherein a diameter of said beam tunnel is greater than a separation
distance between adjacent ones of said polepieces;
wherein said non-magnetic plates each have a respective slot, said
respective slots each providing a respective resonant cavity, said
magnetic polepieces having a notch, said notches coupling said respective
cavities.
4. The focusing structure of claim 3, wherein said beam tunnel intersects
with said respective cavities.
5. An integral polepiece focusing structure for an RF amplification tube,
comprising:
a slow-wave circuit comprising a plurality of magnetic polepieces and a
plurality of electrically conductive non-magnetic plates which are
alternatingly and integrally coupled into a laminate structure;
a means for inducing a magnetic field in said slow-wave circuit having
lines of flux which flow through said magnetic polepieces; and
a beam tunnel provided through said structure, said magnetic polepieces
extending substantially entirely to said beam tunnel;
wherein a diameter of said beam tunnel is greater than a separation
distance between adjacent ones of said polepieces;
wherein said magnetic field induced by said inducing means has an axial RMS
value that varies substantially across a cross section of said beam
tunnel;
wherein said axial RMS value of said magnetic field is greatest at an
outermost portion of said beam tunnel.
6. An integral polepiece focusing structure for an RF amplification tube,
comprising:
a slow-wave circuit comprising a plurality of magnetic polepieces and a
plurality of electrically conductive non-magnetic plates which are
alternatingly and integrally coupled into a laminate structure;
a means for inducing a magnetic field in said slow-wave circuit having
lines of flux which flow through said magnetic polepieces; and
a beam tunnel provided through said structure, said magnetic polepieces
extending substantially entirely to said beam tunnel;
wherein a diameter of said beam tunnel is greater than a separation
distance between adjacent ones of said polepieces;
wherein said slow-wave circuit has a plurality of sides, and further
comprises a planar surface disposed on at least one of said plurality of
sides of said slow-wave circuit, said planar surface having a heat sink
attached thereto.
7. A focusing system for an electron beam within an RF amplification tube,
comprising:
a plurality of magnetic polepieces each having a centrally disposed
aperture;
a plurality of electrically conductive non-magnetic plates alternatingly
and integrally provided with said polepieces, said non-magnetic plates
each having a centrally disposed aperture, said apertures of said
polepieces being aligned with said apertures of said non-magnetic plates
to provide a beam tunnel;
at least one permanent magnet coupled to said polepieces, said magnet
having magnetic flux which flows through said magnetic polepieces to
provide an axial magnetic field within said beam tunnel;
wherein, a slow-wave circuit is provided within said polepieces and said
non-magnetic plates that extends beyond a diameter of said beam tunnel,
said diameter being greater than a separation distance between adjacent
ones of said polepieces.
8. The focusing system of claim 7, wherein said axial magnetic field varies
substantially across a cross section of said beam tunnel.
9. A focusing system for an electron beam within an RF amplification tube,
comprising:
a plurality of magnetic polepieces each having a centrally disposed
aperture;
a plurality of electrically conductive non-magnetic plates alternatingly
and integrally provided with said polepieces, said non-magnetic plates
each having a centrally disposed aperture, said apertures of said
polepieces being aligned with said apertures of said non-magnetic plates
to provide a beam tunnel;
at least one permanent magnet coupled to said polepieces, said magnet
having magnetic flux which flows through said magnetic polepieces to
provide an axial magnetic field within said beam tunnel;
wherein, a slow-wave circuit is provided within said polepieces and said
non-magnetic plates that extends beyond a diameter of said beam tunnel,
said diameter being greater than a separation distance between adjacent
ones of said polepieces;
wherein said axial magnetic field varies substantially across a cross
section of said beam tunnel;
wherein said axial magnetic field has a greatest RMS value at an outermost
portion of said beam tunnel.
10. A focusing system for an electron beam within an RF amplification tube,
comprising:
a plurality of magnetic polepieces each having a centrally disposed
aperture;
a plurality of electrically conductive non-magnetic plates alternatingly
and integrally provided with said polepieces, said non-magnetic plates
each having a centrally disposed aperture, said apertures of said
polepieces being aligned with said apertures of said non-magnetic plates
to provide a beam tunnel;
at least one permanent magnet coupled to said polepieces, said magnet
having magnetic flux which flows through said magnetic polepieces to
provide an axial magnetic field within said beam tunnel;
wherein, a slow-wave circuit is provided within said polepieces and said
non-magnetic plates that extends beyond a diameter of said beam tunnel,
said diameter being greater than a separation distance between adjacent
ones of said polepieces;
wherein at least one of said non-magnetic plates has a respective slot
disposed therein, said respective slot providing a respective resonant
cavity.
11. The focusing system of claim 10, wherein ones of said magnetic
polepieces adjacent said at least one non-magnetic plate have a notch
disposed therein, said notches coupling said respective cavity.
12. The focusing system of claim 11, wherein said beam tunnel intersects
with said cavity.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to microwave amplification tubes, such as
traveling wave tubes or klystrons, and more particularly, to an integral
polepiece RF amplification tube having enhanced gain and transmission.
2. Description of Related Art
Microwave amplification tubes, such as traveling wave tubes (TWTs) or
klystrons, are well known in the art. These microwave tubes, are provided
to increase the gain, or amplify, an RF (radio frequency) signal in the
microwave frequency range. A coupled cavity TWT typically has a series of
tuned cavities which are linked or coupled by irises formed between the
cavities. A microwave RF signal induced into the tube propagates through
the tube, passing through each of the coupled cavities. A typical coupled
cavity TWT may have up to thirty individual cavities which are coupled in
this manner. The meandering path which the RF signal takes as it passes
through the tube reduces the effective speed of the traveling signal so
that it can be operated upon. The reduced velocity wave formed by a
coupled cavity tube of this type is known as a "slow wave."
Each of the cavities is further linked by a beam tunnel which extends the
length of the tube. To produce an amplified RF output signal, an electron
beam must be projected through the beam tunnel. The beam is guided by
magnetic fields which are formed in the tunnel region. The electron beam
will interact with the RF signal to produce the desired amplification. The
bandwidth of frequencies of the resulting RF output signal can be changed
by altering the dimensions of the cavities, and the strength of the RF
output signal can be changed by altering the voltage and current of the
beam.
The magnetic field which is induced in the tunnel region is obtained from
flux lines which flow radially through polepieces from magnets lying
outside the tube region. The polepiece is typically made of magnetic
material, which channels the magnetic flux to the beam tunnel. This type
of electron beam focusing is known as Periodic Permanent Magnet (PPM)
focusing. An RF amplification tube can either utilize an "integral
polepiece" or a "slip-on polepiece." An integral polepiece forms part of
the vacuum envelope extending inward towards the beam region, while a
slip-on polepiece lies completely outside the vacuum envelope of the tube.
When the polepieces form part of the tunnel as well as the cavity wall,
the magnetic flux in the beam region can result in large beam stiffness
values, or .lambda..sub.p /L (where ".lambda..sub.p " is the wavelength of
the plasma frequency of the beam and "L" is the period of the sinusoidal
function of the magnetic field in which the beam propagates), a desirable
condition for focusing beams. For this reason, integral polepiece RF
amplification tubes are preferred over slip-on polepiece tubes.
Klystrons are similar to coupled cavity TWTs in that they can comprise a
number of cavities through which an electron beam is projected. The
klystron amplifies the modulation on the electron beam to produce a highly
bunched beam containing an RF current. A klystron differs from a coupled
cavity TWT in that the cavities are not generally coupled. A portion of
the klystron cavities may be coupled, however, so that more than one
cavity can interact with the electron beam. This particular type of
klystron is known as an "extended interaction output circuit."
A significant problem with RF amplification tubes is the efficient removal
of heat. As the electron beam drifts through the tube cavities, heat
energy resulting from stray electrons intercepting the tunnel walls must
be removed from the tube to prevent reluctance changes in the magnetic
material, thermal deformation of the cavity surfaces, or melting of the
tunnel wall. To remove the heat, copper plates are usually joined to the
portion of the magnetic material that conducts the heat to the heat sink.
The use of copper lowers the thermal resistance of the heat path and more
easily keeps the tunnel temperature below dangerous levels. The minimum
thermal path length in typical cylindrical cavities is the radius of the
cavity.
An additional problem with RF amplification tubes is that it becomes more
difficult to construct them to amplify RF signals in the millimeter
wavelength range of the microwave spectrum, or millimeter waves. These
extremely short wavelength signals require precise tolerances in the
formation of the cavities and the coupling irises. It is well known that
in a periodic microwave structure, an increase in the period-by-period
variation of the inside dimensions (seen by the RF fields), will result in
an increase of RF reflections inside the tube. This, in turn, results in
degraded impedance matches between the tube and the RF input waveguide,
and lower periodicity values than would otherwise exist. These factors
result in reduced gain values achievable by the tube. Thus, as the nominal
dimensions of parts decrease with the higher frequencies, the size of the
period-by-period variations must also decrease.
In prior art integral polepiece RF amplification tubes, magnetic and
non-magnetic parts are usually machined individually, stacked, then brazed
together. In tubes designed to operate at millimeter wavelengths, the
period-by-period dimensional variations are often determined not only by
the tolerances called out for the individual parts, but also by
non-uniformities of the braze regions between the parts. At higher
frequencies, where more periods and hence more parts are usually required,
it becomes more difficult or costly to avoid tolerance build-up along the
stack, especially if copper plates must be added to the polepieces to
improve the thermal conductivity along the cavity wall.
Consequently, integral polepiece RF amplification tubes become less useful
as the operating frequencies and the number of parts increase. More often,
the tube is machined out of a single block of copper using discharge
machining technique to control the dimension variation problem.
Afterwards, a separate magnetic circuit is slipped on and brazed to the
tube if light weight PPM focusing is desired. However, by eliminating the
integral polepiece, and the consequent introduction of magnetic flux at
the tunnel wall, the desirable focusing property of integral polepiece RF
amplification tubes has been lost. The ratio of .lambda..sub.p /L is
significantly reduced, and only higher beam voltages can be focused.
Another consideration with PPM focusing systems is the relationship between
beam tunnel diameter and separation between centers of adjacent
polepieces. Generally, a relatively small diameter beam tunnel is desired
since it presents better interaction impedance with the electron beam,
resulting in greater RF output power and gain. In integral polepiece PPM
focusing systems, the iron of the polepiece can extend towards the beam
axis so as to form part of the beam tunnel or be very close to the beam
tunnel. In such cases, the polepiece geometry typically maintains a ratio
of:
d/P<1
in which d is the diameter of the hole in the iron polepiece (or the beam
tunnel diameter) and P is the separation between centers of adjacent
polepieces. Slip-on polepiece PPM focusing systems often have a ratio of
hole diameter to polepiece separation of greater than one, however, the
interior region of the beam tunnel used by the beam is usually near the
axis of the system.
In focusing an electron beam, the magnetic field strength at the edge of
the beam is of primary significance. Electron beams are often defined in
terms of the ratio of the effective radius of the beam and the beam tunnel
radius, known as the electron beam "fill factor." An electron beam fill
factor of 0.6 is considered typical. PPM focusing systems utilizing the
geometric relationship defined above tend to exhibit very small RMS axial
magnetic field variation across the beam tunnel diameter. While this is
acceptable for ideal electron beams having relatively smooth electron
motion with no radial velocity component, imperfect electron beams are not
so efficiently focused. An imperfect beam may exhibit electron excursions
that impinge on the beam tunnel wall, generating excess heat and reducing
the efficiency of the RF amplification tube.
Beam tunnel size also has an effect on the gain achieved by the RF
amplification tube. Gain of a propagating RF wave in a traveling wave tube
is proportional to the normalized transverse wave number, .gamma.a, where
.gamma. is the radial phase constant of the wave, and a is the radius of
the circuit on which the RF wave propagates, in this case, a is the radius
of the beam tunnel. In PPM focusing systems at high frequencies, a small
beam tunnel radius is considered essential for effective interaction
between the electron beam and the propagating RF wave, and gain generally
decreases when .gamma.a becomes too large. The normalized transverse wave
number is also proportional to 2.pi./.lambda., in which .lambda. is the
wavelength of the propagating RF wave, and is a measure of the size of the
RF wave with respect to the beam tunnel. For large values of .gamma.a, the
RF electric and magnetic fields fall off rapidly away from the beam tunnel
surface. Thus, in actual practice, PPM focusing systems generally select
.gamma.a to be less than 2.2 in order to achieve a useful gain level.
Thus, it would be desirable to provide an integral polepiece RF
amplification tube for amplifying a millimeter wave RF signal having
polepieces extending fully, or at least partially, to the tunnel wall to
provide desirable beam focusing. It would also be desirable to provide an
integral polepiece RF amplification tube having copper plates in contact
with the polepieces along the cavity wall to improve heat removal from the
tunnel wall. It would be further desirable to provide a relatively
inexpensive method of fabricating an integral polepiece RF amplification
tube having the aforementioned features and which eliminates the
deleterious effects of tolerance build-up. It would also be desirable to
provide an integral polepiece PPM focusing system that has greater RMS
magnetic field strength at the outer portion of the beam tunnel for more
efficient focusing of the electron beam.
SUMMARY OF THE INVENTION
Accordingly, a principal object of the present invention is to provide an
integral polepiece RF amplification tube which amplifies a millimeter wave
RF signal, and which has polepieces extending to the tunnel wall for
improved beam focusing.
Another object of the present invention is to provide an integral polepiece
RF amplification tube which amplifies a millimeter wave RF signal, and
which has copper plates in contact with the polepieces along the cavity
wall to improve thermal ruggedness and minimize thermal deformation of the
cavity surfaces, reluctance variation of the magnetic material and melting
of the tunnel wall which could result from high temperature operation.
Yet another object of the present invention is to provide a low cost method
for making an integral polepiece RF amplification tube which eliminates
the deleterious effects of tolerance build-up.
Still another object of the present invention is to provide an integral
polepiece PPM focusing system that has greater RMS magnetic field strength
at the outer portion of the beam tunnel for more efficient focusing of the
electron beam.
In accomplishing these and other objects, there is provided an RF
amplification tube having a laminate structure comprising a plurality of
magnetic and non-magnetic plates which are alternatingly and integrally
formed together. The structure has substantially planar external surfaces
and an internal beam tunnel. A plurality of magnets are provided which
form a magnetic field having lines of flux flowing first through the
magnetic plates then into the tunnel. The planar surfaces are provided on
edges of the structure, and allow for the attachment of planar boundary
heat sinks to the circuit. The non-magnetic plates each have one or more
slots which provides a resonant cavity after attachment of the heat sinks.
The beam tunnel extends through each of the magnetic plates and passes
through each of the cavities, permitting projection of an electron beam
therethrough. The use of planar configuration would be compatible with the
goal of low cost construction, while achieving the needed geometry for the
RF amplification. The non-magnetic plates contributes to removal of heat
from the structure.
In an embodiment of the invention, a focusing system for an electron beam
within an RF amplification tube is provided. The focusing system comprises
a plurality of magnetic polepieces each having a centrally disposed
aperture, and a plurality of electrically conductive non-magnetic plates
alternatingly and integrally provided with the polepieces, the
non-magnetic plates each having a centrally disposed aperture. The
apertures of the polepieces are aligned with the apertures of the
non-magnetic plates to provide a beam tunnel through which the electron
beam travels. At least one permanent magnet is coupled to the polepieces,
the magnet having magnetic flux which flows through the magnetic
polepieces to provide an axial magnetic field within the beam tunnel. The
diameter of the beam tunnel is selected to be greater than a separation
distance between adjacent ones of said polepieces, and the axial magnetic
field varies substantially across a cross section of the beam tunnel. The
axial magnetic field has a greatest RMS value at an outermost portion of
the beam tunnel.
A more complete understanding of the integral polepiece RF amplification
tube for millimeter wave frequencies of the present invention will be
afforded to those skilled in the art, as well as a realization of
additional advantages and objects thereof, by a consideration of the
following detailed description of the preferred embodiment. Reference will
be made to the appended sheets of drawings which will be first described
briefly.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of an integral polepiece RF amplification tube
of the present invention;
FIG. 2 is a partial perspective view of the integral polepiece RF
amplification tube with the magnetic flux lines and the heat flux lines
illustrated;
FIG. 3 is a perspective view of an unassembled, non-magnetic plate with an
exposed pilot hole;
FIG. 4 is an exploded view of the integral polepiece RF amplification tube
of FIG. 1;
FIG. 5 is a cross-sectional view of the interior of integral polepiece RF
amplification tube, as taken through the Section 5--5 of FIG. 2;
FIG. 6 is a partial perspective view of an integral polepiece RF
amplification tube for klystron operation;
FIG. 7 is a sectional side view of an RF amplification tube assembled to an
electron gun and collector; and
FIG. 8 is a graph illustrating a relationship between axial magnet field
strength and normalized radial position for assorted PPM focusing systems.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
This invention provides an integral polepiece RF amplification tube for
amplifying a millimeter wave RF signal having polepieces extending fully,
or at least partially, to the tunnel wall to provide desirable beam
focusing. The integral polepiece RF amplification tube has copper plates
in contact with the polepieces along the cavity wall to improve heat
removal from the tunnel wall. Moreover, the integral polepiece PPM
focusing system has greater RMS magnetic field strength at the outer
portion of the beam tunnel for more efficient focusing of the electron
beam and greater gain. In the description that follows, like numerals are
used to identify individual elements of the invention that are illustrated
in one or more of the figures.
Referring first to FIGS. 1 and 4, an RF amplification tube 10 according to
the present invention is illustrated. The tube 10 is comprised of a
laminate structure having a plurality of non-magnetic plates 18 and
magnetic plates 16 (see FIG. 1) which are alternatingly assembled and
integrally formed together. As seen in FIG. 1, the assembled tube 10 is
elongated and generally rectangular, having end plates 12 disposed on
either end, a first side 23, a second side 25 opposite the first side 23,
a third side 27 and a fourth side 29 opposite the third side 27. As will
be further described below, an electron beam provided in one end of the
tube 10 would travel through a plurality of cavities formed within the
TWT, and exit from an opposite end of the TWT.
Each of the magnetic plates 16 and non-magnetic plates 18 are generally
rectangular. The preferred material for the magnetic plates 16 is iron,
although other magnetic materials could be advantageously utilized. The
magnetic plates 16, also known as polepieces, have a notch 22 disposed at
an edge. The notch 22 shown in the drawings is generally rectangular, and
extends less than halfway through the width of the polepiece. However, it
is anticipated that alternative notch shapes, such as circular, be
advantageously used as well as rectangular.
The notch position for each polepiece 16 could alternate between the edge
corresponding with the first side 23 and the edge corresponding with the
second side 25. As best shown in FIG. 4, the position of the notch 22 in
polepiece 16.sub.1 appears at the first side 23. The next polepiece
16.sub.2 has a notch 22 disposed at the second side 25. The third
polepiece 16.sub.3 would again feature the notch 22 at the first side 23,
similar to that of polepiece 16.sub.1. Alternatively, the notch positions
could all remain on a single side of the TWT 10, or could be a combination
of the two configurations having a portion of the notches 22 disposed at
the first side 23 and a portion disposed on the second side 25. In yet
another embodiment, a single polepiece 16 could have more than one notch
22, such as one at both ends of the polepiece. As will be further
described below, these notches will provide a coupling path for the
neighboring cavities.
The non-magnetic plates 18 are adjacently positioned relative to the
polepieces 16, and alternate with the polepieces. The preferred material
for the non-magnetic plates 18 is copper, although other non-magnetic
thermally conductive materials could be advantageously utilized. Each of
the non-magnetic plates 18 has one or more internal slots 24. Each slot 24
has a generally parallelepiped shape, which extends fully through the
plate 18 from the first edge 23 to the second edge 25. The slot 24 shape
could also be oval in cross-section. Alternatively, the slot 24 could
extend between the third side 27 and the fourth side 29. The slot
direction could also alternate between a first direction extending between
the first and second sides 23 and 25, and a second direction extending
between sides 27 and 29. These slots 24 provide a tuned cavity 26.
It should be apparent from FIG. 4 that with the alternating polepieces 16
and non-magnetic plates 18 integrally formed together, there would be a
continuous path through the tube 10 that passes through each cavity and
crosses over each notch into an adjacent cavity. This path is also visible
in the sectional drawing of FIG. 5.
Extending fully lengthwise through the tube 10 is an electron beam tunnel
14. The tunnel 14 is generally circular in shape and passes through each
of the cavities 26, further linking the cavities. The beam tunnel provides
a path for the projection of an electron beam through the completed
coupled cavity tube 10. With the cavities 26 coupled by the notches 22 as
described above, the tube 10 would function as a coupled cavity traveling
wave tube amplifier. In operation, the electron beam interacts with an RF
signal passing through the coupled cavities. Energy from the beam
transfers to the RF signal, to increase the power of the RF signal.
Each of the polepieces 16 and the non-magnetic plates 18 have edges which
are flush with the first side 23 and the second side 25. As will be
further described below, the first side 23 and the second side 25 provide
a planar surface 32, 32' for attachment of a heat sink 34 (see FIGS. 2 and
6). The third side 27 and fourth side 29 are flush with the other edges of
each of the non-magnetic plates 18 and some of the polepieces 16. However,
individual ones of the polepieces 16 extend outward from the third side 27
and the fourth side 29 to provide ears 36. The combination of the flush
surface 38 (see FIG. 1) and the ears 36 provide a mounting position 38 for
the installation of magnets 42. The magnets 42 as shown in FIG. 2 are
substantially rectangular. However, other shapes of magnets, such as
cylindrical, can be advantageously used.
As shown in FIG. 2, the magnets 42 are disposed within the mounting
positions 38 relative to the TWT 10 so as to provide a magnetic field
having flux lines 44 through the polepieces 16. The flux lines extend
through the polepieces 16, jump across the non-magnetic plates 18 into the
adjacent polepiece 16. The flux lines 44 also cross through the beam
tunnel 14 to provide focusing for the electron beam. The magnetic flux
lines 44 then jump across the space formed by the notch 22, back through
the adjacent cavity 26 and into the first polepiece 16. It should be
apparent that the heat sink surface 32 can be moved closer to the tunnel
14 by changing the shape of the slots 24 and the notches 22, therefore
improving still further the heat handling ability of the tube 10. The
polepieces 16 extend fully to the edge of the beam tunnel 14. It should be
apparent, however, that the beam tunnel 14 may be provided with a thin
coating of thermally conductive material, such as copper, to improve the
thermal handling capability of the TWT 10. The coating would necessarily
be thin enough so as not to disturb the magnetic flux path from the
polepieces 16 to the beam tunnel 14.
Referring now to FIG. 6, there is an alternative embodiment in which the
tube 10 can provide klystron operation. A portion of the magnetic plates
16 are provided without notches. As the electron beam passes through the
tube 10, an electromagnetic field is formed within the cavities 26 which
produces an RF signal. As known in the art, a portion of the cavities 26
can be coupled by the notches 22 to operate as an extended interaction
output circuit for improved bandwidth.
To assemble an RF amplification tube 10 of the present invention, a
laminate structure of generally rectangular, magnetic, and non-magnetic
plates must be formed. Each of the magnetic and non-magnetic plates has a
center alignment hole. A thin-walled molybdenum is inserted through each
of the alignment holes, so that the alternating plates can be aligned
together. Once the plates are assembled they are integrally formed
together into the laminate structure by brazing or other joining
technique. Each of the non-magnetic plates further has a pilot hole 52
extending from the edge associated with the first side 23 to the edge
associated with the second side 25. An exemplary pilot hole 52 in an
unassembled non-magnetic plate 18 is shown in FIG. 3. Once the structure
of magnetic and non-magnetic plates are brazed together into an integral
unit, the pilot holes 52 extend through a width of the structure and
provide a mechanism for cutting out the cavities, as will be further
described below. Alternatively, the laminate structure of magnetic and
non-magnetic plates could be assembled and brazed together first, and the
pilot hole 52 cut through the laminate structure afterward.
The next step is to reduce the exposed edges of the rectangular tube 10
into an approximate shape. It is anticipated that this be done through
conventional milling techniques. Once the sides are squared off, the
desired notches 22 are cut into the sides 23 and 25. The notches extend
entirely across the width of the polepieces 16 and partially extend into
each adjacent non-magnetic plate 18. As known in the art, the preferred
cutting technique is dependent on the desired tolerance requirement.
After the notches 22 are formed, the cavities 26 can be cut out. The
preferred method of cutting the cavities 26 is by using wire electron
discharge machining (EDM). Under this technique, a wire is fed through the
pilot holes 52 to cut away the undesired copper material, leaving the slot
24 without cutting through the cavity wall. This step is repeated to form
each of the cavities 26 in the tube 10. After the cavities 26 are formed,
a continuous path would result from the notches 22 which join the cavities
26.
The wire EDM technique is then used to square off the first side 23 and the
second side 25, providing the heat sink surfaces 32, 32'. The wire EDM
technique can also be used to remove side portions of the polepieces 16
and non-magnetic plates 18, leaving only the exposed ears 36. As desired,
this last step can be performed to leave ears every three polepieces as
shown in FIG. 1, or every two polepieces, as shown in FIG. 2. The
molybdenum tube is also removed by the wire EDM technique, and the tool
used to form the electron beam tunnel 14.
The final step in forming the tube 10 is to provide an entrance and exit
port into each of the end plates 12. These ports provide for the RF signal
to input into and output from the tube 10. The ports can also be formed
with conventional milling or EDM techniques. The finished TWT 10 can then
have heat sinks 34 affixed to the heat sink surfaces 32.
To put the integral polepiece RF amplification tube 10 into use, the tube
must be assembled with other similar circuits into a complete amplifier
assembly. A matching circuit can be added to the finished coupled cavity
tube 10 to match the RF impedance between the RF input port and the tube
itself. The matching circuit is typically machined into a portion of the
coupled cavity tube 10. The tube 10 can then be assembled with other tube
sections as shown in FIG. 7, to an electron gun 62 and an electron beam
collector 64. The electron gun 62 has a cathode 63 which heats up to emit
electrons. The electrons are focused into a beam 66 by the magnetic field
provided in the beam tunnel 14 of the tube 10. The collector 64 receives
and dissipates the electrons after they exit the tube 10. RF input and RF
output terminals are provided for amplification of an RF signal.
It should be apparent to those skilled in the art, that the use of an RF
amplification tube having a laminate structure and generally planar
surfaces would be relatively inexpensive to construct. The copper plates
which form the slots provide additional thermal ruggedness, by conducting
heat from the beam tunnel to the heat sink. The desired geometry for the
millimeter wave frequencies can be accurately obtained without tolerance
build-up.
Since the magnetic field strength, B, on the edge of the electron beam is
the prime consideration for focusing the electron beam, and an imperfect
electron beam has a greater percentage of electron excursions at the outer
radius of the electron beam, it would be advantageous to have a greater
RMS axial magnetic field at the outer radius, than at the inner radius.
This way, the weaker magnetic field at the center of the electron beam
would cause more of the electron beam to have its equilibrium position
moved closer to the beam tunnel wall. By moving more of the electrons of
the electron beam to the outer radial position, enhanced electron
interaction with the RF wave could be achieved over the prior art RF
amplification tubes.
Greater variation in the magnetic field strength could be introduced in the
beam tunnel 14 of the RF amplification tube 10 through selection of the
ratio of polepiece spacing and beam hole diameter. Referring now to the
cross section view of FIG. 5 (not drawn to scale), an RF amplification
tube 10 is illustrated having a beam tunnel 14 with a diameter d and a
separation P between centers of adjacent polepieces 16. As described
above, prior art integral polepiece PPM focusing systems typically
maintain a ratio of d/P of less than one. The inventors have found,
however, that an RF amplification tube having a ratio of d/P of greater
than one would yield increased axial magnetic field variation across the
beam hole cross section, and thus greater gain and beam transmission.
Referring now to FIG. 8, a graph illustrating RMS magnetic field
characteristics of a plurality of electron beams is illustrated. The
ordinate of the graphs gives the ratio of the RMS magnetic field
normalized to the field in the gap provided by the notches 22, illustrated
as B.sub.rms /B.sub.gap. The abscissa of the graphs illustrates the
normalized radial position of the beam, given by the ratio of r/d, where r
is the radial position of the beam within the beam tunnel. Each of the
graphs illustrate magnetic field characteristics for various values of
d/P.
Considering first the uppermost curve, it should be apparent that very
little variation in RMS axial magnetic field occurs across the normalized
radial position of the electron beam. As the ratio of d/P increases,
however the magnitude of RMS magnetic field variation increases
substantially. As a result, a larger percentage of an electron beam will
be found at the radial position of 0.6 (corresponding to the outermost
radial position of an electron beam having a fill factor of 0.6) because
the weaker magnetic field in the center of the beam will tend to shift
electrons outward. Moreover, the beam will be focused more efficiently
because the higher field at the wall of the beam tunnel 14 will tend to
move the electrons inward.
Another advantage of this invention concerns the affect of the beam tunnel
14 with a diameter d on amplification. By decreasing the relative spacing
P between adjacent polepieces, the normalized transverse wave number
.gamma.a would increase above 2.2. While PPM focusing systems are
typically inefficient as the normalized transverse wave number increases
beyond this point, this invention has exhibited significant gain due to
the variations of the axial magnetic field in a millimeter wave TWT having
.gamma.a greater than 3.0.
Having thus described a preferred embodiment of a coupled cavity traveling
wave tube for millimeter wave frequencies, it should now be apparent to
those skilled in the art that the aforestated objects and advantages for
the within system have been achieved. It should also be appreciated by
those skilled in the art that various modifications, adaptations, and
alternative embodiments thereof may be made within the scope and spirit of
the present invention. For example, other precision cutting methods, such
as milling or drilling, can be utilized instead of wire EDM. As known in
the art, the dimensions of the components depend upon the frequency range
of the RF signal to be amplified. These dimensions can be varied
dramatically to provide for alternative RF frequency signals and RF
levels.
Additionally, it should also be apparent that slots 24 could be provided in
polepieces 16 as well as the non-magnetic plates 18, and that notches 22
could be provided in the non-magnetic plates as well as the polepieces, as
desired to produce desired tube characteristics. Multiple slots 24 could
also be formed in individual non-magnetic plates 18 or polepieces 16.
The present invention is further defined by the following claims:
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